Classic latchup, consider yourself luck the processor wasn't destroyed. You
MUST guard your inputs against transients. Period.

Howard McGinnis wrote:

> I have a 16C77A based monitoring system that utilizes the analog input
> channels. 4 of the channels are connected to temperature sensing devices
> that operate on 12 VDC and generate a 1-5 volt analog signal for 32-100
> degrees.
>
> When the temperature sensing devices are connected with power on,
> occaisionally the processor locks up. We're seeing a 7 volt spike
> occaisonally when the devices are plugged in, but no postive proof that
> this is locking the processor up.
>
> The watchdog timer is enabled and should reset the processor as the
> routines that kick the dog do not appear to be working (reception of serial
> input data or timer interrupts). The unit does not respond to the host,
> indicating that the serial i/f is not working and an activity LED no longer
> flashes, possibly indicating that the PIC is no longer processing
> interrupts or is out in left field.
>
> Can an overvoltage condition on the analog input cause the processor to
> head for left field?
>
> Howard
> Howard McGinnis
> hmcginni@digital.net
> Electronic Visions, Inc.
> 1650 Barrett Drive
> Rockledge FL 32955
> (407) 632-7530
> http://ddi.digital.net/~hmcginni
> mcginnis@e-visions.com

--
Rick Farmer             |  916 E. Ogden Ave. #312
Electrical Engineer     |  Milwaukee, Wi. 53202
rfarmer@execpc.com      |  414-273-9740